1. Field of the Invention
This invention relates to an electromagnetic pump utilized as means for increasing air pressure, e.g., a compressor, or means for reducing air pressure, e.g., a reducer or a vacuum pump.
2. Description of the Prior Art
Heretofore, there has been well known in the art an electromagnetic pump, which comprises an electromagnet connected to an AC power source for polarity switching for every half cycle, a reciprocative shaft assembly having a permanent magnet magnetically attracted and repelled by the electromagnet for every polarity switching thereof so as to reciprocate in directions corresponding to the magnetic attraction and repulsion, and a fluid operation chamber wherein the volume thereof increases and contracts with the reciprocation of the reciprocative shaft assembly. The reciprocative shaft assembly used for the prior art electromagnetic pump is as shown in FIGS. 1 and 2 and is disclosed in Japanese Utility Model Publication No. 56-42448.
The example of reciprocative shaft assembly 101 shown in FIG. 1, includes a metal shaft portion with screws 103 and 104 extending from the opposite ends thereof for mounting pump diaphragms. A substantially central portion of shaft portion 102 has two rectangular permanent magnet securement spaces 105 and 106, in which flat and elongate permanent magnets 107 and 108 are held and secured in position. Within this reciprocative shaft assembly 101, magnetic flux is generated around rectangular permanent magnet securement spaces 105 and 106 formed on metal shaft portion 102. A magnetic force from the large amount magnetic flux concentrated in the central portion of shaft portion 102 causes an eddy current to be generated which deteriorates the magnetic properties of the magnetic circuit consisting of the electromagnet and permanent magnets.
The example of reciprocative shaft assembly 201 shown in FIG. 2 includes metal shaft portion 202 with screws 203 and 204 extending from the opposite ends thereof for mounting pump diaphragms. A central portion of shaft portion 202 has large space 205 corresponding to the volume of three permanent magnets. Two permanent magnets 206 and 207 are snugly accommodated in the opposite end portions of space 205 and secured in position. In this example, since vacant space 205 occupies a central portion of space 205, and it has an effect of reducing the chances that an eddy current is formed therein.
However, since magnetic conduction between the poles of permanent magnets 206 and 207 is provided by shaft portion 202 of reciprocative shaft assembly 201 shown in FIG. 2, deterioration of the magnetic performance inevitably occurs. Further, since permanent magnets 206 and 207 are covered by shaft portion 202, the outer size and weight of reciprocative shaft assembly 201 is increased, thereby further deteriorating the start characteristics of the electromagnetic pump.